1. Field of the Invention
The present invention relates to start-stop synchronous communicating method capable of recovering improper synchronization produced when information is transferred through the use of start-stop synchronous communications and to a system using the start-stop synchronous communicating method.
2. Description of the Related Art
As this type of improper synchronization recovering method, there has been known one disclosed in Japanese Patent Application Laid-Open No. 57-53163 (53163/82), for example. According to the disclosure, a frame comprised of information bits all having a logic level identical to an idle state has been used to correct the improper synchronization. A description will now be made of the conventional improper synchronization recovering method referring to FIG. 1. In the following description, one frame will be regarded a comprising one bit set as a start bit (ST), seven bits set as character bits and one bit set as a stop bit (SP). In accordance with the usage of a serial line, the start bit and the stop bit are respectively regarded as a digit "0" and a digit "1". Further, when all of the character bits have a digit "1", the character bits are regarded to represent the idle sate.
When a receiving side enters into a receiving operation while a transmitting side is transmitting an information bit (3a-2) of an information frame (3a), the receiving side interprets a bit (3a-3) having the first trailing edge a start bit. As a result, improper synchronization takes place on the receiving side. The eighth bit (3b-2) as counted from the start bit in the information frame placed to the improper synchronization is interpreted as a stop bit. Since a frame comprising information bits all indicating "1" is used to correct the improper synchronization in the conventional system, bit (3b-2) is represented as "1". Thus, bit (3b-2) is regarded as being identical to the normal stop bit and a frame having a bit range from the bit (3a-3) to the bit (3b-2) is received as a normal frame.
Since information bits in a recovery frame for improper synchronization are all "1" even when the receiving side has entered into an operation for receiving the next frame, the improper synchronization is no longer produced. Thus, since a frame start or frame start pulse can be detected based on a start bit (3c-1) in th next information frame (3c), the next information frame (3c) can be accurately detected.
In this type of conventional improper synchronization correcting system, an improper synchronizing state can be satisfactorily changed to a normal synchronizing state. However, since an alarm is not sounded upon a change, the improper synchronizing state is returned to the normal synchronizing state without any indication of the occurrence of the improper synchronization. Thus, the conventional improper synchronization correcting system has a drawback in that even when improper characters are being received during communications, such reception cannot be determined as improper.